In the current conventional practice of total joint replacement, the concave surface of an ultra high molecular weight polyethylene (UHMW-PE) implant mates and articulates with the convex surface of a hard metallic or ceramic component. For example, in enarthrodial joints such as the hip, a metal or ceramic ball is articulated in match with an UHMW-PE acetabular cup; in ginglymus type joints such as the knee, the hard distal femoral replacement component articulates with a mated UHMW-PE tibial implant. Although UHMW-PE, which was developed and introduced for general use by Dr. John Charnley of England in the late 1960s, has proven to be exceptionally durable, this material is, nonetheless, vulnerable to an unfavorably high rate of wear that may result in a premature failure of the procedure. For the most common total joint coupling currently in use, i.e., cobalt-chrome alloy on UHMW-PE, the polyethylene wear rate is typically on the order of 0.1 to 0.3 mm annually, resulting in the proliferation of billions of wear particles per year. Over the subsequent years, the massive over-loaded accumulation of particulate wear debris, including a substantive percentage of submicron sized particles, gains access to the bone-prosthetic interface and induces an adverse foreign body reaction associated with phagocytic activity and the attendant release of bone resorptive chemical mediators (periprosthetic osteolysis). Structural bone loss due to debris-induced periprosthetic osteolysis is now commonly cited as a major etiological factor in implant loosening and failure of joint replacement procedures. See, e.g., Jacobs, Soc. Biomaterials Trans., April 1998, p. XLVII. See also, Goodman, Id., p. XLVIII; Ramamurti et al., Id., p. 5; Goodman et al., Id., p. 58; Clineff et al., Id., p. 445; Amstutz et al., Id., p. 447. But see, Townley, Ceramic Transactions, Vol. 48, pp. 23-34, 1995 (structure of implant to physiologically stress load remaining bone stock may play part in ameliorating or precluding initial loosening to hinder or preclude osteolysis).
Appropos the present invention, nevertheless, of known art from other researchers, it has been reported extensively that polyurethane or polyurethane-containing polymers have been successfully employed as biocompatible implants for the replacement of tendons and for other body structures such as arteries, veins and so forth. See, e.g., Hsu et al., Soc. Biomaterials Trans., April 1998, p. 72; Labow et al., Id., p. 74; Lee et al., Id., p. 114; McCloskey et al., Id., p. 133; Tanzi et al., Id., p. 176; Archambault et al., Id., p. 251; Santerre et al., Id., p. 254; Weisberg et al., Id., p. 305; Martin et al., Id., p. 306; Tang et al., Id., p. 363; Tang et al., Id., p. 371; White et al., Id., p. 385; Wang et al., Id., p. 393; Huang et al., Id., p. 398; Lee et al., Id., p. 515; Jahangir et al., Id., p. 519; Cohen et al., Id., p. 523. Of course, polyurethanes and polyurethane elastomers are well known as structural components and for other uses outside the field of medicine as well. See, e.g., Jonas, U.S. Pat. No. 5,053,274; Axelrood & Frisch, Rubber Age, Vol. 88, pp. 465-71, 1960.
In a recent preliminary study, it has been additionally reported that six patients received a knee hemi-arthroplasty for unicompartmental osteoarthritis in Norway from the surgical team of Engebretsen et al., utilizing a flowable, prepolymerized polyurethane polymer, which was molded in vivo and subsequently allowed to polymerize to a hardened state to replace only the degraded articular surface of the pathologically involved tibial plateau. See, Rapp, Orthopaedics Today, January/February 1998, pp. 1, 10. However, a polyurethane-containing implant, or for that matter any other plastic type implant material that is contrived to replace only one side of an arthritically degraded and rough bony articulation (hemi-arthroplasty a la the Norwegian method) in opposition to the mirror-imaged equal roughness of the unreplaced joint surface is generally known to be exceptionally vulnerable to rapid abrasive wear degradation and failure of the procedure early on following resumption of weight bearing. Utilizing a plastic material of any sort that articulates in opposition to a bony or cartiligenous surface is not considered to be a viable procedure.
Many years ago, over a span of two years (1960-1961) Charles O. Townley, M.D., utilized a similar in vivo "cured" polyurethane polymer to replace the acetabulum in eleven patients with crippling, severely advanced osteoarthritis of the hip joint. In those cases, however, the polyurethane-containing acetabular implant was used in conjunction with a metallic femoral component to provide a total articular joint replacement. While all of the patients sustained an exceptionally good, pain-free result early on, i.e., over a postoperative follow up time ranging from two to three years, the procedures subsequently failed due to fragmentation of the polyurethane acetabular implant under the duress of normal weight-bearing activities. Looking back upon it, that was an apparent reflection of the primitive nature and structural inadequacy of the polyurethane polymer available for use then, and, the inability to replicate, in vivo, the level of polymerization required to produce a more satisfactorily durable, high density, load-bearing implant. Nonetheless, the early encouraging results of this initial introduction to the concept of a metal-on-plastic total joint arthroplasty strongly supported the rationality of the procedure as a viable mode of treatment, awaiting only the development of a more durable polymer replacement material. Although Charnley's subsequent introduction of in vitro polymerized, high density polyethylene later on in the late 1960s appeared to be the solution to the problem and has, indeed, while becoming an accepted mainstay in the art, extended the life expectancy of total joint procedures for many fruitful years of normal pain free function, the extensive long term clinical experience with this material has demonstrated its time related wear limitations and the associated propensity for latent complications as heretofore cited.
Thus, the current material of choice for prosthetic "socket" sides of joints such as the hip, shoulder, knee, and so forth is UHMW-PE. However, in addition to the foregoing considerations, UHMW-PE cannot be heat-sterilized, and is vulnerable to time-related, on-shelf degradation. Furthermore, it is a rigid plastic material, which is subject to wear and deformation under complex stresses, and to stress-cracking on long term exposure to fluids. Accordingly, there is a need for another suitable plastic material for this application which should result in prolonged longevity in wear and resistance to degradation.
More recently, McGovern et al., Soc. Biomaterials Trans., 25th Annual Meeting (1999) page 452, reported on thermomechanical properties of polyurethane elastomers used as coating and bearing materials; and Drews et al., Id., at 601, reported on the fabrication of a cushion bearing, elastomeric polyurethane composite acetabular cup. Therein, these reported that certain low modulus polyurethanes or polyurethane elastomers had been proposed for total joint arthroplasty since they encourage fluid film lubrication or provide enhanced tribological conditions, which was a "cushion foam bearing" acetabular cup having a low modulus (20 MPa) polyurethane layer introduced into the joint space. See, Auger et al., "Friction and lubrication in cushion form bearings for artificial hip joints," Proc. Inst. Mech. Eng., H-207 (1993) at pages 25-33. However, the cushion foam bearing only has a certain low modulus polyurethane as a layer for articulation in a multilayered composite. As stated in Drews et al., while these materials display promising viscoelastic and tribological properties, they do not possess the fatigue strength needed for long-term bearing implant applications. See also, for example, McClure et al., "Determination of lubricating film thickness for permeable hydrogel and non-permeable polyurethane layers bonded to a rigid substrate with particular reference to cushion form hip joint replacements," Proc. Inst. Mech Eng., H-210 (1996) at pages 89-93.
It would be desirable to overcome/ameliorate such problems. It would be desirable to fulfill such a longstanding lack and long felt need in the art.